The victim was airlifted to Wellington after suffering serious head injuries in the crash, which happened minutes after a brawl broke out outside the pub.
The brawl, between 10-15 people, started just before 2.20am, and within the space of about four minutes, the man was hit by the car, and the driver fled the scene, a police spokeswoman said.
"It was sickening ... pretty awful," said witness Darryl Mallett who had just crossed the City Bridge at the time of the incident.
"I heard a loud bang and then lots of people screaming. The car took off and there was this young guy lying on the grond in the middle of the road."
Mr Mallett, a well-known radio breakfast host with Whanganui station The Hits, had been out for a few drinks in the town centre and was at the bottom of the Durie Hill steps.
"It was horrendous ... it's all a bit of blur after that," he told the Chronicle.
Friends of the victim have been leaving messages of love and support on his Facebook page as he remains in a serious condition in Wellington Hospital's intensive care unit.
"Thinking of you bro, keep on fighting. We all know you're a tough bastard, you got this. Miss you, wishing you and the family the best," one person posted on the victim's Facebook page.
Another said she had been "thinking of you all day" and was "sending love to you and your family".
"Putting all my energy into you brother," another person said.
The police investigation is ongoing, and no charges have yet been laid.
